1. Field of the Art
The present invention relates generally to a sputter cathode assembly and more particularly to a high current rotating sputter cathode assembly in which the power is supplied to the cathode at a point within the sputter chamber.
2. Description of the Prior Art
Direct current (“DC”) reactive sputtering is often used for large area commercial coating applications, such as the application of thermal control coatings to architectural and automobile glazings. In this process, the articles to be coated are passed through a series of in-line vacuum chambers isolated from one another by vacuum locks. Such a system may be referred to as a continuous in-line system, or simply a glass coater.
Inside the chambers, a sputtering gas discharge is maintained at a partial vacuum pressure of about three millitorr. The sputtering gas comprises a mixture of an inert gas, such as argon, with a small proportion of a reactive gas, such as oxygen, for the formation of oxides.
Each chamber contains one or more cathodes held at a negative potential of about 200 to 1000 volts. The cathodes may be in the form of elongated rectangles, the length of which spans the width of the line of chambers. The cathodes are typically 0.10 to 0.30 meters wide and a meter or greater in length. A layer of material to be sputtered is applied to the cathode surface. The surface layer or material is known as the target or the target material. The reactive gas inside the chamber forms the appropriate compound with the target material.
Ions from the sputtering gas discharge are accelerated into the target and dislodge, or sputter off, atoms of the target material. These atoms, in turn, are deposited on a substrate, such as a glass sheet, passing beneath the target. The atoms react at the substrate surface or during passage from the target to the substrate with the reactive gas in the sputtering gas discharge to form a thin film.
The above glass coating process was made commercially feasible by the development of the magnetically-enhanced planar magnetron. This magnetron has an array of magnets arranged in the form of a closed loop and mounted in a fixed position behind the target. A magnetic field in the form of a closed loop is thus formed in front of the target plate. The field causes electrons from the discharge to be trapped in the field and travel in a spiral pattern, which creates a more intense ionization and higher sputtering rates. Appropriate water cooling may be provided to prevent overheating of the target. The planar magnetron is further described in U.S. Pat. No. 4,166,018 which is herein incorporated by reference for everything it teaches.
The rotary or rotating cylindrical magnetron was developed to overcome some of the problems inherent in the planar magnetron. The rotating magnetron uses a cylindrical cathode and target. The cathode and target are rotated continually over a magnetic array which defines the sputtering zone. As such, a new portion of the target is continually presented to the sputtering zone, which eases the cooling problem and allows higher operating powers to be utilized. The rotation of the cathode also insures that the target erosion zone comprises the entire circumference of the cathode covered by the sputtering zone. This increases target utilization. The rotating magnetron is described further in U.S. Pat. Nos. 4,356,073 and 4,422,916, the entire disclosures of which are hereby incorporated by reference.
The rotating magnetrons, while solving some problems, present others. Particularly troublesome has been the development of suitable apparatus for driving and supporting the magnetron in the coating chamber. Conventional rotating cathode bodies consist of a rotating cylinder supported within a fixed housing. The housing is connected with a vacuum chamber in which the sputtering process takes place. In order to maintain the integrity of the vacuum chamber, it is necessary to provide a seal between the rotating cathode body and the fixed housing. Vacuum and rotary water seals have been used to seal around a drive shaft and cooling conduits which extend between the coating chamber and the ambient environment. However, such seals have a tendency to develop leaks under conditions of high temperature and high mechanical loading. Various mounting, sealing, and driving arrangements for cylindrical magnetrons are described in U.S. Pat. Nos. 4,443,318; 4,445,997; and 4,466,877, the entire disclosures of which are hereby incorporated by reference. These patents describe rotating magnetrons mounted horizontally in a coating chamber and supported at both ends.
A preferred seal used to solve the above problems is one which uses a ferro-fluid to make the seal. This consists in part of a fluid suspension of microscopic-magnetic particles in a carrier liquid. The fluid is held in place by a magnetic field provided by an assembly of permanent magnets and steel.
Another troublesome sputtering problem has been an arcing phenomena, which is particularly troublesome in the DC reactive sputtering of silicon dioxide and similar materials such as aluminum oxide and zirconium oxide. As DC power, and therefore voltage, is increased, the charge on the rotating cathodesc tends to build up. Once the charge has built to a certain level, the charge will dissipate by arcing. Insulating materials like silicon dioxide are particularly useful to form high quality, precision optical coatings such as multilayer, antireflection coatings and multilayer, enhanced aluminum reflectors. In addition, when faster sputtering is desired, the power supplied must also be increased, resulting again in undesirable arcing.
Perturbation of the sputtering conditions due to arcing is especially detrimental to a cost effective operation, as any article being coated when an arc occurs will most likely be defective. For instance, the article may be contaminated by debris resulting from the arc, or it may have an area with incorrect film thickness caused by temporary disruption of the discharge conditions. Furthermore, the occurrence of arcs increases with operating time, and eventually reaches a level which requires that the system be shut down for cleaning and maintenance.
One way to avoid the problem of arcing is to avoid using a high DC current and instead to use fluctuating power sources, such as an alternating current (“AC”) source or a square or pulsed DC power source. Oscillating current constantly switches the power supplied to the rotating member, as fast or faster than 50 KHz, constantly relieving the charge build up before it can cause an arc. Arcing is thereby minimized or eliminated.
Utilizing a fluctuating electrical current, however, gives rise to other types of problems. When the rotating cathodes are powered by an oscillating current power supply, any electrically conductive part near the path of the electrical current will be subject to heating via magnetic induction. This is generally not a problem at relatively low current. However, as frequency and/or current are increased, the rate of inductive heating becomes more and more significant and problematic. High frequency and current may be desired because some materials require a higher power density to be sputtered efficiently, such as when sputtering TiO2, SiO2 or Al2O3. Furthermore, to maintain the same power density over a long cathode requires more current, further exacerbating the heating problem. Finally, a higher current and frequency increases the overall sputter rate, and therefore the line speed, of the sputtering operation, resulting in a more efficient production rate.
The electrical induced heating effect is even more of a problem when ferro-fluid seals are used. Ferro-magnetic materials, like the seal, magnify the induced inductive heating effect by focusing the induced magnetic field within themselves. As the current and frequency of the oscillating power supply increases, the seal is heated. If the currency and frequency are high enough, the seal overheats and fails. This failure is usually catastrophic to the sputtering process and thus costly to manufacturing. Additionally, the inductive heating represents a waste of energy and thus reduced efficiency in the sputtering process.
Accordingly, there is a need in the art for an improved sputtering cathode assembly and process which minimizes or eliminates inductive heating when using an fluctuating power supply, thereby facilitating the use of higher oscillation frequencies and currents. This in turn facilitates the more efficient sputtering of materials which require high power densities, such as the reactive sputtering of TiO2, SiO2 and Al2O3, and significantly increases the sputter rate, and thus the line speed, of sputtering operations.